FM Remote Speaker System

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					                                                                                                                                           FM Remote Speaker System
                                                                  National Semiconductor
  FM Remote Speaker                                               Application Note 146
  System                                                          June 1975




  INTRODUCTION
  A high quality noise free wireless FM transmitter receiver      of the VCO is g 0 15V peak A reduction due to the sum-
  may be made using the LM566 VCO and LM565 PLL Detec-            ming network brings the required input to about 0 2V rms for
  tor The LM566 VCO is used to convert the program materi-        g 10% modulation of fc based on nominal output levels
  al into FM format which is then transformer coupled to stan-    from stereo receivers Input potentiometer R1 is provided to
  dard power lines At the receiver end the material is detect-    set the required level The output at pin 3 of the LM566
  ed from the power lines and demodulated by the LM565            being a frequency modulated square wave of approximately
  The important difference between this carrier system and        6V pk-pk amplitude is amplified by a single transistor Q1
  others is its excellent quality and freedom from noise          and coupled to the AC line via the tuned transformer T1
  Whereas the ordinary wireless intercom uses an AM carrier       Because T1 is tuned to fc it appears as a high impedance
  and exhibits a poor signal-to-noise ratio (S N) the system      collector load so Q1 need not have additional current limit-
  described here uses an FM carrier for inherent freedom          ing The collector signal may be as nuch as 40 – 50V pk-pk
  from noise and a PLL detection system for additional noise      Coupling capacitor C8 isolates the transfomer from the line
  rejection                                                       at 60 Hz
  The complete system is suitable for high-quality transmis-      A Voltage regulator provides necessary supply rejection for
  sion of speech or music and will operate from any AC outlet     the VCO The power transformer is sized for peak second-
  anywhere on a one-acre homesite Frequency response is           ary voltage somewhat below the regulator breakdown volt-
  20 – 20 000 Hz and THD is under % for speech and music          age spec (35V) with a 125V line
  program material                                                RECEIVER
  Transmission distance along a power line is at least ade-       The receiver amplifies limits and demodulates the received
  quate to include all outlets in and around a suburban home      FM signal in the presence of line transient interference
  and yard Whereas many carrier systems operate satisfacto-       sometimes as high as several hundred volts peak In addi-
  rily only when transmitter and receiver are plugged into the    tion it provides audio mute in the absence of carrier and
  same side of the 120–240 V power service line this system       2 5W output to a speaker
  operates equally well with the receiver on either side of the
                                                                  The carrier signal is capacitively coupled from the line to the
  line
                                                                  tuned transformer T1 Loaded Q of the secondary tank T1C2
  The transmitter is plugged into the AC line at a radio or       is decreased by shunt resistor R1 to enable acceptance of
  stereo system source The signal for the transmitter is ideal-   the g 10% modulated carrier and to prevent excessive tank
  ly taken from the MONITOR or TAPE OUT connectors pro-           circuit ringing on noise spikes The secondary of T1 is
  vided on component system Hi-Fi receivers If these outputs      tapped to match the base input impedance of Q1A Recov-
  are not available the signal could be taken from the main or    ered carrier at the secondary of T1 may be anywhere from
  extra speaker terminals although the remote volume would        0 2 to 45V p-p the base of Q1A may see pk-to-pk signal
  then be under control of the local gain control The carrier     levels of from 12 mV to 2 6V
  system receiver need only be plugged into the AC line at the
                                                                  Q1A –Q1D operates as a two-stage limiter amplifier whose
  remote listening location The design includes a 2 5W power
                                                                  output is a symmetrical square wave of about 7V pk-pk with
  amplifier to drive a speaker directly
                                                                  rise and fall times of 100 ns
  TRANSMITTER                                                     The output of the limiting amplifier is applied directly to the
  Two input terminals are provided so that both LEFT and          mute peak detector but is reduced to 1V pk – pk for driving
  RIGHT signals of a stereo set may be combined for mono          the PLL detector
  transmission to a single remote speaker if desired              The PLL detector operates as a narrow band tracking filter
  The input signal level is adjustable by R1 to prevent over-     which tracks the input signal and provides a low-distortion
  modulation of the carrier Adding C2 across each input re-       demodulated audio output with high S N The oscillator
  sistor R7 and R8 improves the frequency response to             within the PLL is set to free-run at or near the carrier fre-
  20 kHz as shown in Figure 5 Although casual listening does      quency of 200 kHz The free-run frequency is fo           1 (3 7
  not demand such performance it could be desired in some         R16C13) Since the PLL will lock to a carrier near its free-run
  circumstances                                                   frequency an adjustment of R16 is not strictly necessary
  The VCO free-running frequency or carrier frequency fc de-      R16 could be fixed at 4700 or 5100X Actually the PLL with
  termined by R4 and C4 is set at 200 kHz which is high           the indicated value of C11 can lock on a carrier within about
  enough to be effectively coupled to the AC line VCO sensi-      g 40 kHz of its center frequency However rejection of im-
  tivity under the selected bias conditions with VS e 12V is      pulse noise in difficult circumstances can be maximized by
  about g 0 66 fc V For minimum distortion the deviation          carefully adjusting fo to the carrier frequency fc Adding
  should be limited to g 10% thus maximum input at pin 5
                                                                                                                                           AN-146




C1995 National Semiconductor Corporation   TL H 7442                                                        RRD-B30M115 Printed in U S A
                                      TL H 7442 – 1
FIGURE 1 Carrier System Transmitter




                                      TL H 7442 – 2
 FIGURE 2 Carrier System Receiver




                2
C10 e 100 pF will reduce the carrier level fed to the power         ing the mute in the absence of a carrier when the limiter
amplifier Even though the listener cannot hear the carrier          amplifier output is a series of narrow 7V spikes
the audio amplifier could overload due to carrier signal pow-       The LM380 supplies 2 5W of audio power to an 8X load
er                                                                  Although this is adequate for casual listening in the kitchen
A mute circuit is included to quiet the receiver in the ab-         or garage for hi-fi listening a larger amplifier may be direct
sence of a carrier Otherwise when the transmitter is turned
                                                                    CONSTRUCTION
OFF an excessive noise level would result as the PLL at-
tempts to lock on noise The mute detector consists of a             PC board layout and stuffing diagrams are shown in Figures
voltage doubling peak detector D1Q2C7 The peak detector             3     4 After the receiver board has been loaded and
shunts the 1–2 mA bias away from Q1E without loading the            checked the power transformer is mounted to the foil side
limiter amplifier When no carrier is present the a 4V line          of the board with a piece of fish-paper or electrical insulating
biases Q1E ON via R10 and R11 and the audio signal is               cloth between board and transformer Insulating washers of
shorted to ground When a carrier is present the 7V square               – inch thickness can be used to advantage in holding
wave from the limiter amplifier is peak detected and the            the transformer away from the foil The board is laid out so
resultant negative output is integrated by R9C7 averaged by         that the volume control potentiometer may be mounted on
R10 across C7 and further integrated by R11C6 The result-           either side of the board depending on the desired mounting
ant output of about b4V subtracts from the a 4V bias sup-           to a panel
ply thus depriving Q1E of base current Peak detector inte-          The line coupling coils are available in production quantities
gration and averaging prevents noise spikes from deactivat-         from TOKO AMERICA INC 1250 Feehanville Drive Mount
                                                                    Prospect IL 60056 TEL (312) 297-0070




                                                                                                                  TL H 7442 – 3
                 FIGURE 3 Carrier System Transmitter PC Layout and Loading Diagram (Not Full Scale)




                                                                                                                         TL H 7442 – 4
                   FIGURE 4 Carrier System Receiver PC Layout and Loading Diagram (Not Full Scale)


                                                                3
FM Remote Speaker System
                           ADJUSTMENT
                           Adjustments are few and extremely simple Transmitter car-
                           rier frequency fc is fixed near 200 kHz by R4 and C4 the
                           exact frequency is unimportant T1 for both transmitter and
                           receiver are tuned for maximum coupling to and from the
                           AC line Plug in both receiver and transmitter no carrier
                           modulation is necessary Insure that both units are opera-
                           tive Observe or measure with an AC VTVM the waveform at
                           T1 secondary in the receiver Tune T1 of the transmitter for
                           maximum observed signal amplitude Then tune T1 of the
                           receiver for a further maximum Repeat on the transmitter
                           then the receiver Tuning is now complete for the line cou-
                           pling transformers and should not have to be repeated for
                           either If the receiver is located some distance from the
                           transmitter in use or on the opposite side of the 110– 220V
                           service line a re-adjustment of the receiver T1 may be                                                                                                                                                     TL H 7442 – 5
                           made to maximize rejection of SCR dimmer noise The re-                                                                     FIGURE 5 Overall System Performance
                           ceiver PLL free-running frequency is adjusted by R16 Set                                                          Transmitter Input to Input of Receiver Power Amplifier
                           R16 near the center of its range Rotate slowly in either                                                         The 2 5W audio amplifier provides an adequate sound level
                           direction until the PLL loses lock (evidenced by a sharp in-                                                     for casual listening The LM380 has a fixed gain of 50
                           crease in noise and a distorted output) Note the position                                                        Therefore for a 2 5W max output the input must be 89 mV
                           and then repeat rotating in the other direction Note the                                                         This is slightly less than the g 10% deviation level so we are
                           new position and then center R16 between the two noted                                                           within design requirements Average program level would
                           positions A fine adjustment may be made for minimum                                                              run a good 10 dB below this level at 28 mV input
                           noise with an SCR dimmer in operation The final adjust-
                           ment is for modulation amplitude at the transmitter Connect                                                      Noise rejection is more than adequate to suppress line
                           the audio signal to the transmitter input and adjust the input                                                   noise due to fluorescent lamps and normal line transients
                           potentiometer R1 for a signal maximum of about 0 1V rms at                                                       Appliance motors on the same side of the 110 – 220V line
                                                                                                                                            may produce some noise Even SCR dimmers produce only
                           the input to the LM566 Adjustment is now complete for
                                                                                                                                            a background of impulse noise depending upon the relative
                           both transmitter and receiver and need not be repeated
                                                                                                                                            location of receiver and SCR Otherwise performance is
                           A STEREO SYSTEM                                                                                                  noise-free anywhere in the home Satisfactory operation
                           If full stereo or the two rear channels of a quadraphonic                                                        was observed in a factory building so long as transmitter
                           system are to be transmitted both transmitter and receiver                                                       and receiver were connected to the same phase of the
                           must be duplicated with differing carriers Omit R8 and in-                                                       three-phase service line
                           clude R7 C2 on the transmitter if desired Carriers could                                                         APPLICATIONS
                           be set to 100 and 200 kHz for the two channels Actually
                                                                                                                                            Additional applications other than home music systems are
                           they need only be set a distance of 40 kHz apart
                                                                                                                                            possible Intercoms are one possibility with a separate
                           PERFORMANCE                                                                                                      transmitter and receiver located at each station A micro-
                           Overall S N is about 65 dB Distortion is below about %                                                           phone can serve as the source material and the system can
                           at low frequencies and in actual program material it should                                                      act as a monitor for a nursery room Background music may
                           not exceed % as very little signal power occurs in music                                                         be added to existing buildings without the expense of run-
                           above about 1 kHz                                                                                                ning new wiring




                            LIFE SUPPORT POLICY

                            NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
                            DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL
                            SEMICONDUCTOR CORPORATION As used herein

                            1 Life support devices or systems are devices or                                                                2 A critical component is any component of a life
                              systems which (a) are intended for surgical implant                                                             support device or system whose failure to perform can
                              into the body or (b) support or sustain life and whose                                                          be reasonably expected to cause the failure of the life
                              failure to perform when properly used in accordance                                                             support device or system or to affect its safety or
                              with instructions for use provided in the labeling can                                                          effectiveness
                              be reasonably expected to result in a significant injury
                              to the user

                                       National Semiconductor                              National Semiconductor                                               National Semiconductor                                  National Semiconductor
AN-146




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                           National does not assume any responsibility for use of any circuitry described no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications

				
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